I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a channel structure for communication systems.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known, CDMA has significant advantages over these other techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d and assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein. The CDMA system can be designed to conform to the xe2x80x9cTIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d, hereinafter referred to as the IS-95 standard. Another code division multiple access communication system includes the GLOBALSTAR communication system for world wide communication utilizing low earth orbiting satellites.
CDMA communication systems are capable of transmitting traffic data and voice data over the forward and reverse links. A method for transmitting traffic data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled xe2x80x9cMETHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSIONxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein. In accordance with the IS-95 standard, the traffic data and voice data are partitioned into traffic channel frames which are 20 msec in duration. The data rate of each traffic channel frame is variable and can be as high as 14.4 Kbps.
In the CDMA system, communications between users are conducted through one or more base stations. A first user on one remote station communicates to a second user on a second remote station by transmitting data on the reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on the forward link of the same base station, or a second base station, to the second remote station. The forward link refers to transmission from the base station to a remote station and the reverse link refers to transmission from the remote station to a base station. In IS-95 systems, the forward link and the reverse link are allocated separate frequencies.
The remote station communicates with at least one base station during a communication. CDMA remote stations are capable of communicating with multiple base stations simultaneously during soft handoff. Soft handoff is the process of establishing a link with a new base station before breaking the link with the previous base station. Soft handoff minimizes the probability of dropped calls. The method and system for providing a communication with a remote station through more than one base stations during the soft handoff process are disclosed in U.S. Pat. No. 5,267,261, entitled xe2x80x9cMOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein. Softer handoff is the process whereby the communication occurs over multiple sectors which are serviced by the same base station. The process of softer handoff is described in detail in copending U.S. patent application Ser. No. 08/763,498, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATIONxe2x80x9d, filed Dec. 11, 1996, assigned to the assignee of the present invention and incorporated by reference herein.
Given the growing demand for wireless data applications, the need for very efficient wireless data communication systems has become increasingly significant. An exemplary communication system which is optimized for data transmission is described in detail in copending U.S. patent application Ser. No. 08/654,443, entitled xe2x80x9cHIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEMxe2x80x9d, filed May 28, 1996, assigned to the assignee of the present invention, and incorporated by reference herein. The system disclosed in U.S. patent application Ser. No. 08/654,443 is a variable rate communication system capable of transmitting at one of a plurality of data rates.
A significant difference between voice services and data services is that the former requires a fixed and common grade of service (GOS) for all users. Typically, for digital systems providing voice services, this translates into a fixed and equal data rate for all users and a maximum tolerable value for the error rates of the speech frames, independent of the link resource. For the same data rate, a higher allocation of resource is required for users having weaker links. This results in an inefficient use of the available resource. In contrast, for data services, the GOS can be different from user to user and can be a parameter optimized to increase the overall efficiency of the data communication system. The GOS of a data communication system is typically defined as the total delay incurred in the transfer of a data message.
Another significant difference between voice services and data services is the fact that the former imposes stringent and fixed delay requirements. Typically, the overall one-way delay of speech frames must be less than 100 msec. In contrast, the data delay can become a variable parameter used to optimize the efficiency of the data communication system.
The parameters which measure the quality and effectiveness of a data communication system are the total delay required to transfer a data packet and the average throughput rate of the system. Total delay does not have the same impact in data communication as it does for voice communication, but it is an important metric for measuring the quality of the data communication system. The average throughput rate is a measure of the efficiency of the data transmission capability of the communication system.
A communication system designed to optimize transmission of data services and voice services needs to address the particular requirements of both services. The present invention provides a channel structure which facilitate transmissions of data and voice services.
The present invention is a novel and improved channel structure for use in communication systems. The present invention provides for two sets of physical channels, one for the forward link and another for the reverse link, to facilitate communication of a variety of logical channels. The physical channels comprise data and control channels. In the exemplary embodiment, the data channels comprise fundamental channels which are used to transmit voice traffic, data traffic, high speed data, and other overhead information and supplemental channels which are used to transmit high speed data. In the exemplary embodiment, the forward and reverse traffic channels can be released when the remote stations are idle to more fully utilized the available capacity. The control channels are used to transmit control messages and scheduling information.
It is an object of the present invention to provide a channel structure which supports voice services and data services. In the exemplary embodiment, the traffic channels comprise fundamental and supplemental channels. The fundamental channels can be used to transmit voice traffic, data traffic, high speed data, and signaling messages. The supplemental channels can be used to transmit high speed data. In the exemplary embodiment, the fundamental and supplemental channels can be transmitted concurrently. In the exemplary embodiment, to improve reliability (especially for signaling messages) the fundamental channels are supported by soft handoff.
It is another object of the present invention to provide a channel structure which maximizes the throughput rate of a communication system. In the exemplary embodiment, the supplemental channels transmit at one of a plurality of data rates. The data rate is selected based on a set of parameters which can comprise the amount of information to be transmitted, the transmit power available for the remote station, and the required energy-per-bit. The data rate is assigned by a scheduler such that the system throughput rate is maximized.
It is yet another object of the present invention to provide a channel structure which optimizes transmissions from multi-cell and multi-carrier. In the exemplary embodiment, the power levels of all base stations in the active set of the remote station are measured periodically during a communication. The multi-cell xcex94 power levels are transmitted to the base stations which use the information to transmit high speed data from the xe2x80x9cbestxe2x80x9d set of base stations, thereby increasing capacity. In addition, the power levels of all carriers are also measured periodically and the multi-carrier xcex94 power levels are transmitted to the base stations. The base stations can use the information to increase the power level of weak carriers or to reassign the remote station to a new carrier assignment.
It is yet another object of the present invention to provide a channel structure which minimizes power consumption and increase system capacity. In the exemplary embodiment, the remote station operates in one of three operating modes which comprise the traffic channel mode, the suspended mode, and the dormant mode. If the period of inactivity since the termination of the last transmission exceeds a first predetermined threshold, the remote station is placed in the suspended mode. In the exemplary embodiment, in the suspended mode, the traffic channel is released but the state information is retained by both the remote station and the base station and the remote station monitors the paging channel in the non-slotted mode. Thus, the remote station can be brought back to the traffic channel mode in a short time period. If the period of inactivity exceeds a second predetermined threshold, the remote station is placed in the dormant mode. In the exemplary embodiment, in the dormant mode, the state information is not retained by neither the remote station nor the base station but the remote station continues to monitor the paging channel in the slotted mode for paging messages.
It is yet another object of the present invention to provide a channel structure which minimizes processing delay for high speed data transmissions. In the exemplary embodiment, the control data are transmitted over control frames which are a fraction of the traffic channel frame. In the exemplary embodiment, the data rate request by the remote station and other information are transmitted by the remote station using a control channel frame format which minimizes the processing delay between the time a data rate request is made to the time of actual transmission at the assigned data rate. In addition, the present invention provides for erasure-indicator-bits for both the forward and reverse links which can be used in place of NACK RLP frames defined by the IS-707 standard.